Semiconductor devices



Sept. 28, 1965 c. w. MUELLER 3,208,924

SEMICONDUCTOR DEVICES Filed March 17, 1961 2 Sheets-Sheet 1 11 IINVENTOR.

g/mas I'll/11115115? United States Patent 3,208,924 SEMICONDUCTOR DEVICES Charles W. Mueller, Princeton, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Mar. 17 1961, Ser. No. 96,422 9 Claims. (Cl. 204-143) This invention relates to an improved method of making improved semiconductor devices. More particularly, .it relates to an improved method of making semiconductor devices such as diodes, which include a rectifying junction.

"In the fabrication of semiconductor junction devices it is often important to control the area of the rectifying junction. For example, in tunnel diodes it is desirable to reduce the area of the P-N junction in order to decrease peak current and control the peak-to-valley current ratio of the device. For a discussion of tunnel diode parameters such as peak current, valley current,, and the like, see article by H. S. Sommers Jr., Tunnel Diodes as High Frequency Devices, Proc. IRE, July 1959.

Semiconductor devices such as diodes and the like have been made by alloying a pellet of electrode material which induces given conductivity type to the surface of a semiconductor wafer of opposite conductivity type. The pellet melts during the alloying process, and dissolves a portion of the semi-conductor wafter material. When the pelletwafer assemblage is cooled, the dissolved wafer material precipitates as an extension of the wafer crystal lattice. The precipitated region contains a sufficient amount of the pellet conductivity type-determining material to be converted to said given conductivity type. A rectifying P-N junction is thus formed between the given conductivity type precipitated region and the opposite conductivity type bulk of the wafer. In order to reduce the area of the P-N junction thus produced, smaller electrode pellets are often utilized. However, electrode pellets smaller than 5 mils in diameter are difficult to handle in mass production. Furthermore, it is difiicult to attach an electrical lead to such small pellets.

It is an object of this invention to provide an improved method of making improved semiconductor junction devices.

Another object is to provide an improved method of controlling the area of a P-N junction.

Still another object is to provide an improved method of making improved tunnel diodes.

But another object is to provide an improved method of controlling the peak current in tunnel diodes.

These and other objects of the invention are accomplished by preparing a given conductivity type semiconductive wafer with two opposing major faces and a surface zone of opposite conductivity type on one major face. An electrode pellet is attached to the opposite conductivity type surface zone so thatthe pellet makes an ohmic contact to the zone. The surface zone of the wafer is then removed except for at least part of the portion immediately beneath the electrode pellet.

The invention and its-advantages will be described in greater detail in connection with the accompanying drawing, in which:

FIGURES 1-6 are cross-sectional views illustrating successive steps in the fabrication of a semiconductor device according to the invention; and,

ice

FIGURE 7 is a schematic'view of a method of etching a semiconductor junction device according to one embodiment of the invention.

The invention will be described in connection with the fabrication of a germanium tunnel diode, but it will be understood that the invention is equally applicable to other crystalline semiconductive materials, such as silicon, germanium-silicon alloys, and semiconductive compounds such as indium phosphide, gallium arsenide, and the like. The conductivity type of the different regions of the device may be reversed. Furthermore, the other types of semiconductor junction devices, including two-terminal devices such as conventional diodes and PNPN diodes, three-terminal devices such as triode transistors and unipolar transistors, and four-terminal devices such as tetrodes, may be similarly fabricated with a junction of Example A crystalline semiconductive wafer 10 of given conductivity type is prepared with two opposing major faces 11 and 12, as shown in FIG. 1. The exact size, shape, material, and doping of wafer 10 is not critical in the practice of the invention, and is adjusted for the particular application desired. 7

A thin surface zone 13 of wafer 10 adjacent to one major'face 11 is converted to opposite conductivity type by any convenient method, for example, by diffusion of an opposite conductivity type-determining material into face 11 of wafer 10. A rectifying barrier or junction 14 is thus formed between surface zone 13 and the bulk of wafer 10.

In this example, wafer 10 consists of P-type germanium, and contains sufiicient gallium to have about 8X10 to 5 X 10 charge carriers per cm. Wafer 10 may suitably be 25 mils square and 2 mils thick. Since the bulk of wafer 10 is P-type, the surface zone 13 must be converted to N-type conductivity by the incorporation of suitable donors. Surface zone 13 may conveniently be formed by flooding face 11 of wafer 10 with a melt (not shown) consisting of gms. tin-lead solder and 2 gmsg'germanium arsenide. A portion of the semiconductive wafer material is thus dissolved from face 11 of wafer 10. On

cooling the melt, this dissolved wa-fer material, together with some of the arsenic of the melt, is deposited on wafer face 11 in the form of an N-type surface layer 13. The melt is then decanted from the wafer. The thin arseniccontaining N-type zone 13 thus formed in wafer 10 is advantageously of the order of 0.1 mil in thickness.

Wafer 10 is now bonded to a metallic support 15, so that surface zone 13 is uppermost, as shown in FIG. 3. The support 15 is preferably made of a metal or alloy which is sufiiciently inert to withstand the action of any etchants subsequently utilized. In this example, the support 15 consists of a gold-plated alloy of nickel-ironcobalt, such as Kovar, Fernico, and the like.

Referring now to FIG. 4, an electrode pellet or dot 16 is attached to N-type zone 13 by fusing or surface alloying. The pellet or dot 16 may be made of material which is electrically inert with respect to wafer 10. In this example, lead, tin, and lead-tin alloy may be utilized for the electrode 16, since these materials are electrically inert with respect to germanium. Alternatively, the electrode dot 16 may consist of or contain donor material which induces the same conductivity as surface zone 13.

In this example, since zone 13 is N-type, the dot '16 may consist of lead and a small amountof a donor such as antimony. The connection'betwccn pellet 16 and surface zonei13 of wafer is thus ohmic in character.

Next, an electrical connection is made to electrode dot 16,v for example, as shown in FIG.5, by means of a metal point 17, which advantageously is made of the same'metal or alloy as support 15. The electrical connection 17 is particularly useful when the device is subsequently electrolytic-ally. etched. The surface zone 13 of-wafer 10'is then removed, except for at-le'ast part of the portion of zone 13 which. is immediately beneath electrode 16, leaving theetched wafer 10 with a surface zone 13"of reduced surface area, and hence a junction 14 of reduced area, as shown in FIG. 6. The'rernovalof aportion of surface zone 13.may be accomplished, for example, by

' junction area 14' is less than the area of electrode 16.

One method of controllably reducing the area of junction 14 in wafer lll-is showntin FIG. 7. The support 15 for the wafer 10 and the electrical connection to clot '16 may conveniently be united by means of an insulating ringls' to form a low-capacitance low-impedance mount or case, "as. described in my application Serial No. 844,663, filed October 6,1959, now US. Patent No. 3,001,113, andassigned to the assignee of theinstant invention. Theconnection 17 to electrode 16 consists of an apertured metal'plate' 19'having a'prong .or lobe 17-within its aperture. The-prong 17 is'bent within the'aperture to form the connection to electrodepellet. 16. The assemblage of the'wafer 10 and its supportor case is immersedin an etching tank 70 containing a suitableetchant 71 and a cathode 72,-which may, for-"example, be platinum or carbon. The'etching. bathin'this example conoccur'whn the dot contains"aconductivity;type'determining material andforms a junction 'withr-thewafer."

-Another advantage of this method is that gthe composition of the dot canbereadi'ly varied to giv e the desired mechanical properties of strength andrresistance for elevated temperatures, since it is-,no't necessary for a doping agent to be present in the. dot. For example,

when the semiconductive wafer'is of hi-conductivity type v germanium or silicon,- and theelectrode'pelletor dot 16 is employed to form an-alloyedPN: junction, the use of low-melting acceptors, such as-indiumand gallium, limits the maximum operating temperature ofthedevice In contrast, ini'the" method of this invention,;.afP-'type region or zone maybe formed on'an 'N-typeiwafer,"and thepellet 16 allowed to: the P-type. zonenmay consist of lead or gold, which-will Withstandihighertemperaturesithan indium.

Various modifications and varia-tions rmay be made" without departing from thespiritJandQscopeJ-ofthe invention. For example, the ohmic contact'matcrial 16 maybe deposited on surface zone 13lbyevaporation; Instead.of

preparing a wafer of .given conductivity type and converting a surface zone thereoftoopposite conductivity type; a thin'slice ofasemiconductive-ingot maybetreated to convert a'zoneadjaccnt one'face of the slice to op i posite; conductivity type. Theslice isthfen readily 'diced sists of a weight percent aqueous solution of potassium hydroxide; The wafer 10- and itssmount or-support is connected as the anode of theietching bath. The etching circuit also includes a double-pull double-throw switch 73,'having acenter otf position, amilliameter '74, and a variable resistance or rheostat 75in series with a source of direct current, such as a battery 76. A curve tracer 78 having its own power. supply is connected between plates 15 and 19 of the devicescase.

In operation, the switching circuit is energized by periodically closing switch 73 for'short periods of time, of the order of 20 seconds. The current through the semiconductor device is adjustedby means of the variable resistance 75 to be. about 250 milliampe'res atthebeginning of the etching'cycle. After each period of etching, switch 73' is thrown in the opposite direction soLthat the peak current of the device-can be read on curve tracer 78. This cycle of etching the unit and reading thepeak current is repeated at number of times, during which the variable resistance 75' is adjusted so that'the etching current is gradually decreased to about 20 milliamperes. When the peak current, as'measured'on curve tracer 7 8, is reduced to the desired value, for example 5 milliamperes, the etching process is halted. The-entireetching cyclemay thus be completed ina few minutes. The device is then removed from the etching bath, washed in deionized water, dried, and hermetically sealed.

Tunnel diodes thus fabricated have exhibited a current into a'plurality ofwafer's. This procedure insuresigreater uniformity in'the composition: of -thesurfaceione-of the wafers. I

Instead of switching theetching-circujt' on -and'otfby hand, adjusting the variable resistance andobserving the curve tracer until the desired'value; of peak current is'attained, more sophisticatedxmethods may be utilizedto obtain automation of'the etching process-'forrmass production. An automatic control circuit-"79lhaving its own power supply'may be substituted for'the curve'tracer 78 and'used to'control switch 73. Thecontrol circuit79 is preset to sample the diode current-at periodic intervals with a constantcurrent. If'sa unit having apeakcurrent of 5' milliamperes is desired, the'constant' samplingcurrent is set at Stnilliamperes. As longas the pealccurrent is above 5 'milliamperes, the-voltage .drop'acrossthe 'diode is about 80 'tomillivolts. Whenthe junction areaof the diode has been sufiiciently 'reducedjtogiveaflpeak current of 5 milliamperes, the. diode will switch to a higher voltage state in :which the. voltagedrop across the diode is'about 400'rnillivolts. The control circuit 79 is arranged so that-this increase/in voltage: cankeep switch 73-open and thus'stopany further reduction in the area of'the' device. junction. v

What .is-claimed .is:

g 1. The method of fabricatinga semiconductor device comprising:

preparing. a given conductivity type semiconductive wafer with two opposing majorrfaoes a surface zone of opposite"conductivity 'type on one" entiremajor face, and a rectifying barrier between. said surface zone andsaid Wafer;

attaching an electrodezto said surface zone so thatsaid electrode 1. makes an ohmic connection thereto; removing from saidone face: allof said'surface "zone 7 except" for the portion immediately beneath said elecjtrode;and-' removing a. portionof saidwaferaroundeaid' electrode.

2. Theme'thod of fabricating a semiconductordevice comprising:

.preparing a given conductivity typescmiconductive wafer with two opposing-major'faces,.a surface zone of opposite conductivity type on one entiremajor face, and a rectifying 'barrierbetwecn said'surface zoneandsaid wafer;

' bonding an electrode :pellet: tosaid'surface 'zone so 'that said pellet makes an ohmic connectionthereto; electrolytically etching away from said; one face all of said surface zone except for the portion immediately beneath said electrode pellet; and

removing a substantial portion of said wafer from the entire said one face around said electrode pellet.

3. The method of preparing a semiconductor device comprising:

preparing a given conductivity type semiconductive wafer with two opposing major faces, a surface zone of opposite conductivity type on one entire major face, and a rectifying barrier between said surface zone and said wafer;

alloying an electrode pellet to said surface zone so that said pellet makes an ohmic connection thereto;

electrolytically etching away from said one face all of said surface zone except for the portion immediately beneath said electrode pellet; and

electrolytically etching away a substantial portion of said wafer from the'entire said one face around said electrode pellet.

4. The method of fabricating a semiconductor device comprising:

preparing a given conductivity type semiconductive wafer with two opposing major faces, a surface zone of opposite conductivity type on one entire major face, and a rectifying barrier between said surface zone and said wafer;

fusing an electrode pellet to said surface zone so that said pellet makes an ohmic connection thereto; immersing said wafer in an electrolytic etching bath and attaching said wafer to the anode of said bath;

electrolytically etching away from said one face all of said surface zone except for the portion immediately beneath said electrode pellet; and

electrolytically etching away a substantial portion of said wafer from the entire saidone face around said electrode pellet so that said one major face slopes downward from the periphery of said electrode pellet.

5. The method of fabricating a semiconductor device comprising:

preparing a given conductivity type semiconductive wafer with two opposing major faces, a surface zone of opposite conductivity type on one entire major face, and a rectifying barrier between said surface zone and said wafer;

evaporating on a portion of said surface zone a quantity of material capable of making an ohmic connection thereto; immersing said wafer in an electrolytic etching bath and attaching said wafer to the anode of said bath;

electrolytically etching away from said one face all of said surface zone except for the portion immediately beneath said evaporated material; and

electrolytically etching away a'substantial portion of said wafer from said one face around said evaporated material so that said one major face slopes downward from the periphery of said evaporated material to form a mesa including said evaporated material and the remainder of said rectifying barrier.

6. The method of fabricating a semiconductor device comprising:

preparing a given conductivity type semiconductive wafer with two opposing major faces, a surface zone of opposite conductivity type on one entire major face, and a rectifying barrier between said surface zone and said wafer;

bonding said wafer to a support so that said opposite type surface zone is uppermost;

attaching an electrode pellet to said surface zone so that said pellet makes an ohmic connection thereto; immersing said wafer in an electrolytic etching bath and attaching said wafer to the anode of said bath; and,

electrolytically etching away said surface zone except for a portion thereof immediately beneath said electrode while monitoring the peak current of said junc- 5 prising:

preparing a given conductivity type semiconductive wafer with two opposing major faces, a surface vzone of opposite conductivity type on one entire major face, and a P-N junction between said surface zone and the remainder of said wafer; bonding said wafer to a support so that said opposite type surface zone is uppermost; alloying an electrode pellet to said surface zone so that said pellet makes an ohmic connection thereto; immersing said wafer in an electrolytic etching bath and attaching said wafer to the anodeofsaid bath;

electrolytically etching away from said'one face all of said surfacezone except for avportion immediately beneath said electrode pellet and electrolytically etching away a substantial portion of said wafer from the entire said one face around said electrode pellet so that said one major face slopes downwardfrom the periphery of said electrode pellet while monitoring the peak current of said junction,vthe area ofsaidremaining portion of said surface zonebeing reduced.

until a predetermined value of said peak current is attained. 8. The method of fabricating a tunnel diode com-' prising:

preparing a given conductivity type germanium wafer with two opposing major faces, a surface zoneof opposite conductivity type on one entire major face, and a rectifying barrier between said surface zone and the remainder of said-wafer;

bonding the other one of said major faces of said wafer to a support;

attaching an electrode pellet selected from the group consisting of lead, tin, and lead-tin alloys to said surface zone so that said pellet makes an. ohmic connection thereto;

immersing said wafer in an electrolytic etching bath andelectrically connecting said wafer to thetanode of said bath; and,

electrolytically etching away said surface zone except for a portion thereof immediately beneath said electrode; and,

electrolytically etching away a substantial portion of said wafer from the entire said one face around said electrode pellet so that said one major faceslopes downward from the periphery of said electrode pellet. 9. The method of fabricating a tunnel diode comprising:

sisting of lead, tin and lead-tin alloys to saidtsurface' zone so that said pellet makes an ohinic connection thereto; immersing said wafer in a potassium hydroxideetching bath and electrically connecting said wafer to the anode of said bath; electrolytically etching away from said one face all of said surface zone except for a portion immediately beneath said electrode pellet, and monitoring the peak current of said junction while electrolytically etching away a substantial portion of said wafer from the entire one face around said electrode pellet so that one major face slopes downward from the periphery of said electrode pellet, the area of said remaining portion Of said surface zone being reduced until a predetermined-valued said peak current is attained. 7

ReferencesCited by the Examiner UNITED STATES PATENTS Solty-s 317-237 j; FOREIGN PATENTS 1,228,285 1 33/60- France. r I 158,915 5/57wSweden.

OTHER REFERENCES,

Jordan et a1. 204--1 43 Technical Bulletin (II) 1484.5, v01; 4, No. 7, December v Herbert 204-143 1961. 1 q t, Stump 148-].5 X IBM, .Etching PN' Junctions, IBM Technical Disclo- Rediker 204,1-43 1 sure Bulletin, vol. 2,,No. 3, October-1959f. Tiley 204-143 v Y L if 204 143 JOHNH. MACK, Primary Examine" MARCUS U. LYONS,-Examiner.

, Dunn: Semiconductor Device ,lFa bricatifon, IBM 

1. THE METHOD OF FABRICATING A SEMICONDUCTOR DEVICE COMPRISING: PREPARING A GIVEN CONDUCTIVITY TYPE SEMICONDUCTIVE WAFER WITH TWO OPPOSING MAJOR FACES, A SURFACE ZONE OF OPPOSTIE CONDUCTIVELY TYPE ON ONE ENTIRE MAJOR FACE, AND A RECTIFYING BARRIER BETWEEN SAID SURFACE ZONE AND SAID WAFER; ATTACHING AN ELECTRODE TO SAID SURFACE ZONE SO THAT SAID ELECTRODE MAKES AN OHMIC CONNECTION THERETO; REMOVING FROM SAID ONE FACE ALL OF SAID SURFACE ZONE EXCEPT FOR THE PORTION IMMEDIATELY BENETH SAID ELECTRODE; AND REMOVING A PORTION OF SAID WAFER AROUND SAID ELECTRODE. 